Sen. Claire McCaskill (Mo.) knows exactly where she needs to be positioned. And she has done about as good a job as possible to sell herself to Show-Me State voters as an “independent” Democrat who doesn’t always agree with her party or her president.

But even veteran party operatives agree that the Senator, identified early on as one of the election cycle’s more vulnerable incumbents, remains in a precarious position. More accurately, perhaps, McCaskill looks like a very weak incumbent who will need some help from the top of the ticket to earn a second term.

“Claire has done a good job of trying to establish herself as an independent Democrat. That’s why she is even in the ballgame,” said one Democrat who thinks that she could win if everything falls her way but readily acknowledges that the odds are stacked against her.

McCaskill’s main problem certainly isn’t her opponents.

Rep. Todd Akin, former Missouri Treasurer Sarah Steelman and businessman John Brunner aren’t the kind of top-tier recruits that would normally make an incumbent shudder with dread.

Akin, from northeast Missouri, isn’t well-known outside his Congressional district and may be too conservative even for Missouri Republicans. Brunner, a businessman with no political experience, laid an egg when he announced his candidacy. And Steelman, whose base in reliably Republican and very conservative southwestern Missouri could be important both in the primary and general election, still has to prove herself as both a fundraiser and a campaigner.

Multiple polls show McCaskill running about even with the three Republicans and drawing 38 percent to 43 percent of the vote.

While an inexperienced observer might figure that those numbers show she has an even-money chance of winning a second term, longtime observers will see a very different situation. They will note that after almost six years in office, far less than half of the electorate is inclined to vote for her against largely unknown opponents — a clear sign of McCaskill’s weakness.

In fact, the Senator’s numbers are reminiscent of those of defeated Senators such as Rick Santorum (R-Pa.) and John Sununu (R-N.H.), both of whom showed considerable weakness early on in ballot tests and insisted that the race would change when voters started to focus on the choice. Of course, that never happened.

McCaskill’s albatross right now may well be President Barack Obama, who lost the state very narrowly four years ago and seems primed to lose Missouri by a far larger margin later this year.

Much like other states, the Show-Me State is looking like a starkly polarized battleground, and that isn’t good for Obama, McCaskill or the Democratic Party.

Savvy political observers talk about the increasing importance of “outstate,” a reference to parts of Missouri not in the Kansas City or St. Louis media markets.

“Traditionally,” one Democratic observer told me recently, “Democrats try to hold down their losses outstate and win big in St. Louis. That doesn’t work any longer. The presidential election is a particular problem. Elections get nationalized, and outstate Missouri is growing, delivering more votes for Republicans.”

If Obama loses Missouri, a likely outcome now, even after the president’s numbers have climbed nationally, McCaskill is unlikely to win. She may get the votes of some who vote against Obama, but not many, possibly a point or even two. But that’s probably a best-case scenario for her.

McCaskill’s prospects would rise, of course, if Republicans were to nominate a weaker general election candidate for president, such as Santorum. But even that might not save her because the kind of suburban Republicans who could not stomach Santorum would quickly return to the GOP column below the presidential race to elect someone who would check Obama during the Democrat’s second term.

With personal issues and a public record that offered effusive early praise for Obama, McCaskill simply has too much baggage. She must try to make her election a referendum on her Republican opponent, but that won’t be easy given the president’s performance, the public’s low opinion of Congress and her own issues.

McCaskill, in fact, is something of an accidental Senator. She was fortunate to face then-Sen. Jim Talent (R) in 2006, President George W. Bush’s second midterm election and a year when many otherwise formidable Republicans lost because of public dissatisfaction with the president’s policy on Iraq.

Talent would have beaten McCaskill in almost any other year, and the Democrat must now face voters in a dramatically different political environment than the one that existed in 2006.

McCaskill is an energetic campaigner and appears frequently on TV and emphasizes her political independence and commitment to political reform. Indeed, a press release from her office last week crowed about new National Journal ratings that showed the Senator to be in the “moderate middle” and bragged about her “strong support for the Keystone XL pipeline” and “work across the aisle to cap federal discretionary spending.”

But Missouri voters are likely to look past those talking points (and McCaskill’s new ad that cites “special interests” attacking her), and polling continues to suggest that McCaskill’s party, and her president, will be an albatross around her neck.

Republican insiders are already counting Missouri as a Senate pickup. It’s hard to argue with their reasoning, which is why I have moved Missouri from a pure tossup to a contest that now tilts Republican.